SOLVED. Favorite children's book with Merlin character [s]

[M.G.]

One of my favorite books from the 1970s involved a boy and a girl who find an old man sleeping inside a tree trunk who turns out to be Merlin. He takes them into a cave and leads them into a magic world. I think the title had the word SECRET in it, like The Secret Cave or The Secret Door or something like that. Anyone know this book?

[bb.]

Is it...The Hidden Cave by Ruth Chew?

[M.G.]

Yes! That's it! Thank you.

[bb.]

You're welcome. :3

[Lobstergirl, au gratin]

A popular book - shelved from this query:

https://www.goodreads.com/topic/show/...

[Rainbowheart]

Yup, lots of people remember The Hidden Cave.